A description is given of a prior art apparatus for measuring a thickness of a semiconductor layer.
Suppose, for example, as shown in FIG. 12, a sample 11 comprising a plurality of semiconductor thin films (laminated films) 2, 3 and 4 disposed on a semiconductor substrate. A light beam 5 having a certain wavelength irradiates the surface of the sample 11 at incident angle .theta.. Numerals 6, 7, 8 and 9 designate respectively primary reflected light components at surfaces of the thin films 2, 3 and 4 and the substrate 1. The film thicknesses and the refractive indices of the thin films 2, 3 and 4 are represented respectively as (d.sub.1, n.sub.1), (d.sub.2, n.sub.2) and (d.sub.3, n.sub.3) and the refractive index of the substrate 1 is represented as n.sub.s. In this case, the reflected light components 6, 7, 8 and 9 at the surface of the respective thin film layers 2, 3 and 4 and the substrate 1 have phase differences due to their optical path lengths and these composite light beams interfere with each other. Namely, the optical path difference .delta.i between the reflected light component 6 at the surface of the uppermost layer of the sample and the reflected light component at an interface between an i-th layer and an (i+1)-th (i=1, 2, 3) layer, both from the uppermost layer, is represented as follows: ##EQU1## where n.sub.o is a refractive index and .theta..sub.o is an incident angle of light on the uppermost surface of a wafer. Therefore, the thicknesses of respective thin films 2, 3 and 4 can be obtained by analyzing the interference waveform of the reflected beams composed of the respective reflected light components 6 to 9 having these optical differences .delta.i.
In order to measure the thicknesses of laminated films, a method of analyzing a wavelength dispersion interference waveform of reflected interference light (hereinafter referred to as an interference spectrum) is employed. However, since the interference spectrum is complicated when the sample has a laminated structure with plural layers, a Fourier analysis method of analyzing the reflection spectrum is employed. More particularly, Fourier-transformation spectroscopy having a high utilization efficiency of light energy and therefore superior in analysis of optically measured data is employed in an apparatus for measuring the above-described reflection spectrum. FIG. 11 is a typical view showing a prior art apparatus for measuring semiconductor layer thickness employing a Fourier-transformation spectroscope, which is disclosed in Japanese Published Patent Application 3-110405.
As shown in the figure, infrared light passing through a semiconductor crystal is emitted from a light source 10, is collimated to be a parallel beam by a collimating mirror 12 and enters into a Michelson interferometer 13. The Michelson interferometer 13 includes a beam splitter 14 dividing the parallel beam into two beams of transmitted and reflected light, a fixed mirror 15 reflecting the transmitted beam from the beam splitter 14, a moving mirror 16 reflecting the reflected beam from the beam splitter 14 and a driver 17 moving mirror 16 at a constant speed in the arrow directions in FIG. 11.
The beams reflected by the fixed mirror 15 and the moving mirror 16 return to the beam splitter 14 and are synthesized as a modulated interfering light beam in which the optical path difference corresponding to the traveling distance of the moving mirror 16 varies with time. A part of the modulation interference light beam is output from the Michelson interferometer 13 and introduced into a reflection photometry system 18. The beam entering into the reflection photometry system 18 is reformed by an aperture 19 to have a predetermined area which is reflected by a plane mirror 20 to irradiate a predetermined area of the sample 11. The light irradiating to the sample 11 is subjected to the interference according to film construction of the sample 11 as described above. The light beam reflected by the sample 11 advances, changing its direction at a plane mirror 21, and is collected by a collecting mirror 22 to enter into a photodetector 23 in which the intensity of reflected interference light is detected. The intensity of the light beam detected at the photodetector 23 includes interference components according to the film construction of the sample 11 and is modulated with the traveling distance accompanying with the constant speed moving of the moving mirror 16 in the above-described Michelson interferometer 13. The interference beam intensity modulated with the distance is transformed by Fourier transformation with the traveling distance, resulting in a wavenumber dispersion spectrum of the light under measurement. This is the principle of so-called Fourier transformation optical spectrometry.
In the thus obtained reflected light spectrum, the film interference component of the sample 11 is superposed on the light intensity distribution which is dependent on the light source 10 in the apparatus and the transmission characteristics of all optical components. Therefore, by dividing the reflected light spectrum with film interference by the reflected light spectrum which is measured by providing a comparative sample such as a semiconductor substrate having no film construction in place of the sample 11, the film interference component can be separated from the reflected light spectrum. Information on the film thickness can be obtained directly from the film interference spectrum. Or, the film thickness can be analyzed from the waveform of the spatial interference intensity distribution (hereinafter referred to as Spatialgram) shown in FIG. 13, which is obtained by executing a reverse Fourier transformation of the film interference spectrum.
In the graph of FIG. 13, the abscissa represents traveling distance of the moving mirror 16 and the ordinate represents reflected light interference intensity. In the Spatialgram of FIG. 13, there appear burst peaks 24 to 26 where all beams interfere with each other and increase their strength at points where the optical path difference of the reflected light components at respective layer interfaces represented by the formula (1) coincides with an optical path difference due to the traveling position of the moving mirror 16. The distance between the respective bursts corresponds to the optical path difference of the reflected light components at the layer interfaces. In the example of FIG. 13, the burst peak 24 corresponding to the reflected light component 6 at the surface of the sample 11 (refer to FIG. 12) appears as a center burst peak and symmetrically at the left and right thereof the reflected light components 7, 8, and 9 of respective layers produce respective side burst peaks 25 to 27. Provided that the distances from the center burst peak 24 to the respective side burst peaks are Li (i=1, 2, 3), an optical path difference .delta.i of the respective reflected light components coincide with 2Li as a sum of going path and return path to and from the moving mirror 16. Therefore, the following equation is obtained from the above-described formula (1): ##EQU2## Here, since the refractive index n.sub.j and incident angle .theta. are known, respective layer thicknesses d.sub.i can be calculated from the distances Li between the bursts which are obtained from the Spatialgram.
Next, a description is given of the operation of the apparatus for measuring a semiconductor layer thickness. Generally, a light beam collecting system shown in FIG. 14(a) is employed as a reflection photometry system. This is to enhance utilization efficiency of light energy in the reflection photometry system. As a result, the signal to noise ratio (SN ratio) of a light detected signal is enhanced, the number of times of photometry integration is reduced which shortens the photometry time. As shown in FIG. 14(a), however, since incident angle of the collected beam is actually distributed continuously with a certain incident angle as a center value and the light beams 5'a and 5'b in the collected light beam incident on the sample 11 have different incident angles .theta.a and .theta.b, differences also appear in the optical path lengths of the respective beams advancing in the film as shown in the figure. This results in many interference waveforms overlapping with each other in the film interference waveform accompanying the distribution of incident angle and turbulence unfavorably occurs in the film interference waveform, further resulting in a problem in analyzing thin films or multi-layer films. Then, in Japanese Published Patent Application 3-110405, by employing as a beam irradiating the sample 11 a parallel beam in a reflection photometry system, as shown in FIG. 14(b), all light rays in the beam have the same optical path lengths and no turbulence occurs in the film interference waveform. This results in quite excellent photometry performance in measuring thin films and multi-layer films.
By the way, in a method for measuring a film thickness from the analysis of the above-described film interference spectrum, the thin film measurement limit (d.sub.limit) is mainly determined by a photometric wavenumber range (.DELTA..nu.cm.sup.-1), which is represented as follows: ##EQU3## Here, n designates a refractive index of the film. However, unless the transmitting or reflecting conditions of the light beam in all the optical elements of the spectroscope and in the measurement sample meet the photometric wavenumber range .DELTA..nu., a spectroscope having a certain photometric wavenumber range .DELTA..nu. cannot be obtained, resulting in difficulty in measuring a wide wavenumber range.
As an example, a description is given of application to measuring a semiconductor film thickness by a combination of optical parts employed in Fourier-transformation infrared spectroscope (hereinafter referred to as FT-IR spectroscope). In FIG. 11, a nichrome lamp is used as light source 10, potassium bromide (KBr) is used as beam splitter 14 and a photodetector employing mercury cadmium telluride (hereinafter referred to as MCT detector) is used as photodetector 23. In FIG. 12 showing a film composition of the sample 11, a semiconductor film 2 comprising an Al.sub.0.45 Ga.sub.0.55 As layer having a refractive index and a thickness (n.sub.1, d.sub.1)=(3.45, 0.42 microns), a semiconductor film 3 comprising an Al.sub.0.15 Ga.sub.0.85 As layer having (n.sub.2, d.sub.2)=(3.56, 0.09 microns), a semiconductor film 4 comprising an Al.sub.0.45 Ga.sub.0.55 As layer having (n.sub.3, d.sub.3)=(3.45, 1.55 microns) and a semiconductor substrate 1 comprising a GaAs substrate having a refractive index n.sub.s of 3.62 are employed. FIG. 15 shows a fundamental reflection spectrum of GaAs semiconductor substrate, which is obtained by measuring a wavenumber over the whole range of a spectroscope by FT-IR spectroscope having the above-described combination. As shown in FIG. 15, the sensitivity can be obtained in a range approximately from 5000 cm.sup.-1 to 600 cm.sup.-1.
FIG. 16 shows a reflection spectrum, which is obtained by measuring the interference spectrum of the above-described three layer semiconductor film with the FT-IR spectroscope. In the figure, there is a waveform accompanying the film interference. FIG. 17 shows a Spatialgram, which is obtained by taking out and transforming by reverse Fourier transformation, a wavenumber range from an arrow A (4300 cm.sup.-1) to an arrow B (800 cm.sup.-1) having photometric sensitivity in FIG. 16. Judging from FIG. 17, regardless of a waveform obtained by measuring a three-layer film, three side burst peaks do not appear and only a single side burst peak corresponding to the third layer appears.
According to the formula (3), in the photometric wavenumber range .DELTA..nu.=4300-800=3500 cm.sup.-1, the thin film measurement limit d.sub.limit (3500) is 0.42 microns, which is equal to the thickness of the first layer semiconductor film 2, showing that it is difficult to distinguish the first layer from the second layer.
Then, a description is given of an example which adopts the most suitable combination of optical systems capable of measuring wavenumber range in a wider. A tungsten halogen lamp is employed as light source 10, calcium fluoride is employed as beam splitter 14 and an MCT detector is employed as photodetector 23. FIG. 18 shows a fundamental reflection spectrum of a GaAs substrate employing the optical system of this combination. As shown in FIG. 18, it is admitted that there is sensitivity in a range from about 11500 cm.sup.-1 to 2500 cm.sup.-1. The limit at higher wavenumber side is sensitivity limit of the MCT detector and the limit at lower wavenumber side is determined by the wavelength of light from the tungsten halogen lamp. FIG. 19 shows an interference spectrum of the semiconductor film of the above-described three layer structure, which is obtained by the optical measurement employing the optical system of this combination. FIG. 20 shows a Spatialgram, which is obtained by taking out and transforming by reverse Fourier transformation, a wavenumber range from an arrow A (11000 cm.sup.-1) to an arrow B (2800 cm.sup.-1) having photometric sensitivity shown in FIG. 19. According to the formula (3), in the photometric wavenumber range .DELTA..nu. of 8200 cm.sup.-1, the thin film measurement limit d.sub.limit (8200) is 0.18 microns and the measurement limit (measurement sensitivity) is enhanced. However, since the second layer of the sample 11 has a thickness that is half of the measurement limit (d.sub.2 =0.09 microns), the first and the second side burst peaks overlap with each other and only the third side burst peak clearly appears.
The prior art apparatus for measuring a semiconductor layer thickness is constructed as described above, and there is a limit to optical measurement over a wide wavenumber range by Fourier spectroscopic photometry employing a combination of present optical materials, resulting in a problem that the thicknesses of thin semiconductor films cannot be measured.